This invention relates to a hydraulic brake booster provided with an auxiliary or secondary fluid pressure supply.
Many existing hydraulic brake boosters are provided with a secondary pressure source which may be used to actuate the booster during failure of fluid communication to the booster from the primary pressure source. Normally, the secondary pressure source is a fluid pressure accumulator which stores fluid generated during normal actuation of the booster for use during the aforementioned failure of the primary pressure source. Existing brake boosters, such as the boosters disclosed in U.S. Pat. No. 3,638,528 owned by the assignee of the present invention, provide actuation of the secondary brake pressure source upon overtravel of the spool valve. However, since actuation of the secondary pressure source in these boosters is dependent upon travel of the spool valve, the secondary pressure source is not actuated if contamination or other failure of the spool valve prevents it from moving. Furthermore, since the secondary pressure source is normally a fluid pressure accumulator containing a finite amount of fluid pressure, the booster mechanism must be sealed during actuation by the secondary pressure source to prevent fluid pressure from leaking from the booster around the lands of the spool valve to the booster ports.